Not much is known about Gerdeth, including how exactly Thea and Blaise are related to her - 'Aunt' could be an honorary title, such as in the case of Aunt Ursula. At one point, the cousins lived with Gerdeth, but were forced to move out, most likely due to Blaise's antics with human boys, resulting in her and Thea being expelled from school. Whilst still living with Gerdeth, Blaise stole a bezoar stone from an ibex's stomach for use in forbidden spells.
Aunt Gerdeth is briefly mentioned by Blaise and Thea. When Thea intends to place a love spell on her soulmate, Eric Ross, so that he will fall in love with a human girl and thus avoid persecution from the Night World, Blaise mentions having one of the key ingredients to the spell - a bezoar she took from Gerdeth whilst they were living with her. When Thea is later deciding whether to drink from the Cup of Lethe to erase her memories of the Night World - thus making her a lost witch and allowing her to be with Eric - she thinks about Gerdeth and how much it will hurt to have to forget about her (although luckily, Blaise is able to switch the potion, allowing Thea to keep her memories).
- Gerdeth is not mentioned on the family tree as being a Harman, nor is she stated as being a Harman in the books. Therefore, it's possible Gerdeth is related to Thea and Blaise through one of their fathers.
- Spellbinder (mentioned only)